Fuses, circuit breakers and circuit interrupters are designed to respond to detected power faults, which, in most cases, corresponds to disconnecting a source from its load (in such cases, an appliance or the like). The disconnect may take a variety of forms such as a melted fusing link, a mechanically opened contact or an electronically opened driver. Advances in these forms have raised the mechanism of disconnection to a well known art.
Apart from the disconnect type, all such devices have the common requirements of sensing circuit currents and reacting when abnormal conditions are detected. As a consequence, the method of determination of normal/abnormal conditions assumes a primary role. Early devices relied upon thermal, magnetic, pneumatic and other physical properties or structures to establish a limit or range of normal/abnormal current. The subsequent inclusion of logic and microprocessor circuits allowed such determinations to advance in complexity by many orders of magnitude. Such advances have introduced a wide range of features.
For instance, overcurrent, underfrequency and undervoltage limits (U.S. Pat. Nos. 4,371,947 and 4,672,501) for primary use in feeder/transmission networks used single equation solutions for trip-times, limits and other constraints. By definition, networks allow an essentially unlimited number and type of loads making any solution representative of the network constraints, as opposed to the load specifics. This approach has persisted to the lowest levels of the power transmission tree--the residential wall outlet.
Intermediate level protectors, typically residential and industrial circuit breakers, offer a wide range of functions. The inclusion of phase faults, ground faults and load restoration (U.S. Pat. No. 4,694,373) added additional sensing features. Load type-specific parameters (U.S. Pat. Nos. 4,967,304 and 4,864,287 and 4,694,374), such as trip-time curves, field excitation limits and pressure sensing broadened the variables. Reliability indicators (U.S. Pat. Nos. 4,958,252 and 4,780,786) of contact condition was provided by storing trip/wear data history. Various forms of security (U.S. Pat. Nos. 4,945,443 and 4,870,531) against setpoint tapering was provided. More functions (U.S. Pat. Nos. 5,136,458 and 5,113,304 and 5,038,246 and 4,991,042) were added, including remote communication links, simulated thermal memories, interchangeble rating plugs, fault magnitude as response modifiers and interlocked selection algorithms. The useful features named above were well-conceived but, again, were defined within the constraints of a network solution.
Attempts to deal with a specific fault type (U.S. Pat. No. 5,121,282), such as an arcing fault, require detailed analysis of an often location-specific fault. Such analysis is generally not possible in the presence of other accompanying conditions. Another complicating factor in such analysis is the wide variety of load (appliance) types, constructions and functions. Clearly, detailed knowledge and understanding of the exact appliance in use is required for the most appropriate protection scheme. Such an approach (U.S. Pat. No. 4,996,625) asserts this mode of solution but only within the secondary circuits of a specific device.
The existing body of art has addressed the problem of electrical safety, particularly the interruption of fault currents, within the context of a transmission network where, over a long enough time frame, given a large enough number of devices in use, all current levels from zero to maximum will be encountered. While such logic is required of large power transmission networks, it imposes severe and unnecessary limitations when applied at the lowest levels of the network, typically the residential wall outlet. At this level, electrical failures are not statistical events with unknown origins and consequences; but rather they are as logical as the normal operation expected of the devices. Failure analysis can be absolute, provided there is enough of the device left to analyze. Cause and effect is always present, often with clear warning events preceding the actual final damage. Increasingly complex apparatus, instruments, and appliances not only operate in more complex sequences, they can also fail in more complex and subtle ways. It is therefore apparent that the existing body of knowledge and art suffer from the disadvantages listed in paragraphs (a) through (e) below.
(a) There is little attention given to the method or form by which present interrupters determine normal/abnormal currents as it directly relates to the specific appliance in use. The performance characteristics of existing interrupters are carefully defined, but only with respect to themselves, thus requiring the appliance to fail in a manner appropriate to detection. Given the wide range of available appliances, it follows that no single-solution interrupter can provide effective protection against all except the most conspicuous faults. Responsibility falls to the user or specified to select an interrupter, usually based solely on its trip rating, with little or no detailed knowledge of the device to be protected.
(b) It is widely known that relatively minor changes in current within an appliance, which are often the first warning signs of problems, are extremely difficult to distinguish from normal fluctuations. Such fluctuations often originate in the fact that various types of appliances react quite differently to changes in applied line voltage, even when such changes are well within normal limits. No attempt is made to correlate applied voltage with current under normal conditions. Additional variables which may affect the appliance, such as external loading, are similarly not taken into account. The resulting changes in current can easily swamp subtle, underlying changes in current indicative of a fault. One of the most common examples of this effect is a faulty wire connection within an appliance which, as a resistive function of its mechanical fault, begins to heat. The rise in internal temperature of the junction further raises its resistance. This type of self-destructive cycle often continues until fire results in destruction of the device itself, and often, worse. During the early stages of such an event, total current through the circuit would drop (caused by the voltage drop across the faulty connector), making detection by existing interrupters extremely difficult and unreliable.
(c) The issue of where and how an interrupter is most appropriately located in a circuit is not addressed. Although it is known that many electrical faults involve cords, extensions, plugs, outlets, and the like in addition to internal failures of various types within the appliance, interrupters are generally found only at extreme ends of the circuit, such as at a power distribution panel of the supply or perhaps at a fuse or mechanical breaker within the appliance. Considering the discussion in (b) above, physical placement of an interrupter is critical. The existing range of physical embodiments of interrupters severely limits such flexibility.
(d) No process or form is represented to allow for the consideration of relative changes in operating characteristics of a given appliance over time. Such changes often include warm-up cycles, long and short-term current drift, current noise, variation in timing cycles, and others. In many cases, these types of changes are very advanced precursors of developing fault conditions. The lack of qualitative judgment in existing interrupters makes such considerations ill-conceived.
(e) No clear information pathways from appliances to interrupters exist. Following the discussions in (a) through (d) above, it follows that the logical source of expert specifications relating to the appliance is the manufacturer. Such data is often internally available from most manufacturers in the form of test data, design limits, quality control limits, failure analysis, and the like. Additional data is available from various industrial safety organizations and agencies. In combination, such pathways would allow an interrupter to perform as an optimum safety device.